This invention relates to the production of grey scale images using pixellated exposure devices such as an array of light emitting diodes (LEDs) or liquid crystal shutters.
Electronic image recording apparatus of a type comprising a line exposure array stationed in light exposing relationship to a photosensitive material and comprising a plurality of linearly spaced apart light emitting diodes are well known in the art. Means are generally provided for effecting a relative displacement between the light exposure array and the photosensitive material in a direction transverse to the longitudinal axis of the exposure array so as to effect an exposure of the entire surface of the photosensitive material.
Line exposure arrays of the aforementioned type comprising a plurality of light emitting diodes aligned in spaced apart relation along the longitudinal axis thereof generally provide for a nonuniform line exposure as a result of those portions of the photosensitive material immediately opposite the areas between the light emitting diodes receiving substantially less exposure than those areas of the photosensitive material immediately opposite the light emitting diodes. This non uniform line exposure results in visible underexposed stripes extending across the photosensitive material in the direction of relative displacement between the photosensitive material and the line exposure array. The visibility of such stripes can be reduced but not entirely eliminated by minimising the spacing between the light emitting diodes. The degree to which the spacing between the light emitting diodes can be minimised, however, is limited by practical constraints since adjacent light emitting diodes must also be insulated from each other. This insulation requirement results in a minimum degree of spacing between the light emitting diodes being required thereby making it impossible to entirely eliminate some minimum degree of spacing between the diodes.
It has been proposed to eliminate "unexposed lines" caused by the spacing of LEDs by oscillating the LED head; arranging the LEDs in separate rows which are combined electronically as disclosed, for example, in U.S. Pat. Nos. 3,827,062, 4,096,486, and 4,435,064 and Japanese Patent No. 60-175065; the particular selection of the LED element shape as disclosed, for example, in U.S. Pat. Nos. 4,435,064 and 4,589,745 and the use of Selfoc lens arrangements in combination with LEDs as disclosed, for example, in U.S. Pat. Nos. 4,318,587 and 4,447,126.
One can distinguish two types of imaging using LED bars, namely bi-level and continuous tone imaging. In the former, each pixel of the photosensitive medium experiences either maximum or zero exposure by an element of the LED array, and the image comprises dots of maximum optical density on a background of minimum optical density. In contrast, continuous tone imaging requires that each pixel receive an exposure that is continuously variable, or variable over a sufficiently large number of discrete levels as to mimic a continuous variation. The latter type of imaging is needed in areas such as high-quality colour reproduction and requires control of the exposure parameters with a degree of precision not achieved in the prior art, in turn, involving the solution of problems not recognised in the prior art. These include transient turn on-and-off effects, source wavelength variation effects and pixel shape and spacing effects, which are addressed by the present invention.
The prior art has mostly involved bi-level imaging, and has been concerned primarily with eliminating element-to-element variations in the output energy of the exposing device, and to minimising unexposed gaps between the elements. For the purposes of continuous tone imaging, especially on high-resolution media such as silver halide films, it is found that much more sophisticated controls and compensations are necessary. What is ultimately important is the developed density in the imaging media as perceived by the human eye, and this is a function of the exposure energy density experienced by the media. For media such as silver halide films, it is a non-linear function. In practice, the eye can detect deviations in transmission density of 1% or perhaps even somewhat less. The methods and apparatus of the prior art do not give this level of precision. When attempts are made to image silver halide film in continuous tone using a linear LED array, the resulting images are frequently distorted by the presence of lines of high or low density running in the direction perpendicular to the long axis of the array. The problem can be traced to non-uniformities in the energy density experienced by the film on a microscopic scale (i.e., over areas too small to be resolved by the human eye), combined with the non-linear relationship of exposure energy to image density.